rust/timer: Define NANOSECONDS_PER_SECOND binding as u64

NANOSECONDS_PER_SECOND is often used in operations with get_ns(), which
currently returns a u64.

Therefore, define a new NANOSECONDS_PER_SECOND binding is with u64 type
to eliminate unnecessary type conversions (from u32 to u64).

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250414144943.1112885-6-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>